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FLEXO Magazine : September 2010
Technologies & Techniques The advent of LCDs (liquid crystal displays) marked the end of the dominance of CRT, yet the technique to calibrate and profile LCDs remained essentially the same. Today ’s displays are largely based on LED (light-emitting diode) technology, including LED backlit, RGB LED and white LED devices. Self-emissive displays and OLED (organic LED display) are coming to market as well. Some display manufacturers are even starting to look at five- or six-color LED technology. The desire for displays with a wider gamut and greater dynamic range is driving this technology forward at a hectic pace. According to ICC chairman, Tom Lianza, “The typical press technology might take several years to be developed and adapted, yet display technology is undergoing a revolution every 18 months or so.” This poses a distinctive challenge to color management software: keeping up with the brisk advance in display technology may require a change in measurement devices (such as the typical RGB filtered colorimeter). Improvements include new white point calibrations based on CCFL. Another adaptation includes reflectance (spectral) characteristics of any new display technology in the calibration of the measure- ment device at the time of manufacture. Support of sRGB gamma is also a new trend, as opposed to the traditional 2.2 gamma curve. The slightly different shape of the sRGB gamma curve lends itself to more ac- curate representation of images or video, many of which are already in the sRGB space. This is really aimed at low-end systems or the use of applications that are not ICC-savvy. Sometimes a software breakthrough is required, for example, the use of iterative profiles for displays. This has long been the purview of output (printer) profiles, but it is now possible to apply this mathematical process to displays. It is a way to apply smoothing curves and stabilize the profile. Another interesting development is the use of brightness adaptation based on CCAM02 (CIE Color Appearance Model), which is a technique that attempts to maintain color appearance between different conditions (i.e . a light booth and a display) and bridge the gap that can occur between human vision and measured data. X-Rite advocates the wider use of reflectance data as a step toward a spectral profiling engine. There is also a push in the ICC for .2 (dot 2) specifications for profiles to include spectral data, without which it is impossible to handle ambi- ent light in profiles. Ambient Light ProfiLes In the years I have spent helping people with color-man- aged workflow, users often tell me, “I need to edit every profile I create. ” T hese users aren’t being overly critical and here’s why: the light source used to create the profile and the light source used to evaluate the color are not the same. It makes sense that when light conditions do not match, the profile must be edited. Most output profiles (printers) are built for a D50 light source, which has the same spectral distribution curve as natural sunlight. See Figure 2. D50 can be simulated with filtered light. But the typical light booth, while still 5,000K, does not use filtered light or has the same spectral distribution curve as D50. Now see Figure 3. This difference can cause metamerism, the nature of color to look different under differ- ent lighting conditions. People who edit each profile are often correcting for metamerism from two lighting conditions. Sometimes you do not have the luxury of knowing in ad- vance under what specific light source the final print will be viewed; it’s also possible that it will be viewed under numerous light sources. In that case, D50 may be the best compromise. Some spectrophotometers have a little appreciated fea- ture: a light diffuser that allows it to act as a spectroradiom- eter and read ambient light. This ambient light reading can then be applied to ICC profiles. It is very useful in a few ways: Matching monitor profiles to light booths and building ambi- ent light profiles for output devices. Using the light source from a light booth for your monitor is a great way to enhance the accuracy of soft proofs. You can measure the color temperature and luminance for the illumi- nant in the light booth with your spectroradiometer and apply the results to your monitor profile. This helps balance the white point and overall look and feel between the hard proof in the light booth and the soft proof on the monitor. Ambient light profiles for output (printers) are created to capture the ambient conditions under which color output is evaluated, typically a light booth. Many light booths or day- light simulators are deficient in UV content. Some may have mercury spikes or a less continuous spectrum; others a more highly regulated power source. All of these factors can add more metamerism when comparing a light booth to D50. One way to combat this effect is to build the output profile for your specific viewing condition. figure 2. spectral curve of D50. figure 3. spectral curve of 5,000K light booth. 46 FLeXO september 2010 www.flexography.org FLX_Sept2010_mech.indd 46 9/1/10 9:24 AM